Mass spectrometry analysis techniques are generally carried out under conditions of high vacuum. However, various types of ion sources used to generate ions for MS analyses operate at or near atmospheric pressures. Thus, those skilled in the art are continually confronted with challenges associated with transporting ions and other charged particles generated at atmospheric or near atmospheric pressures, and in many cases contained within a large gas flow, into regions maintained under high vacuum.
Various approaches have been proposed in the mass spectrometry art for improving ion transport efficiency into low vacuum regions. For example, FIGS. 1A-1B are two schematic depictions of mass spectrometer systems 1-2 which utilize an ion transport apparatus to so as to deliver ions generated at near atmospheric pressure to a mass analyzer operating under high vacuum conditions. As one example, analyte ions may be formed by the electrospray technique by introducing a sample comprising a plume 9 charged ions and droplets into an ionization chamber 7 via an electrospray probe 10. For an ion source that utilizes the electrospray technique, ionization chamber 7 will generally be maintained at or near atmospheric pressure. Although an electrospray ion source is illustrated, the ion source may comprise any other conventional continuous or pulsed atmospheric pressure ion source, such as a thermal spray source, an APCI source or a MALDI source.
In the systems 1-2 illustrated in FIGS. 1A-1B, the analyte ions, together with background gas and partially desolvated droplets, flow into the inlet end of a conventional ion transfer tube 15 (e.g., a narrow-bore capillary tube) and traverse the length of the tube under the influence of a pressure gradient. Analyte ion transfer tube 15 is preferably held in good thermal contact with a heating block 12. The analyte ions emerge from the outlet end of ion transfer tube 15, which opens to an entrance 27 of an ion transport device 5 located within a first low vacuum chamber 13. As indicated by the arrow, chamber 13 is evacuated to a low vacuum pressure by, for example, a mechanical pump or equivalent through vacuum port 31. Under typical operating conditions, the pressure within the low vacuum chamber 13 will be in the range of 1-10 Torr (approximately 1-10 millibar), but it is believed that the ion transport device 5 may be successfully operated over a broad range of low vacuum and near-atmospheric pressures, e.g., between 0.1 millibar and 1 bar.
After being constricted into a narrow beam by the ion transport device 5, the ions are directed through aperture 22 of extraction lens 14 so as to exit the first low pressure chamber 13 and enter into an ion accumulator 36, which is likewise evacuated, but to a lower pressure than the pressure in the first low pressure chamber 13, also by a second vacuum port 35. The ion accumulator 36 functions to accumulate ions derived from the ions generated by ion source 10. The ion accumulator 36 can be, for example, in the form of a multipole ion guide, such as an RF quadrupole ion trap or a RF linear multipole ion trap. Where ion accumulator 36 is an RF quadrupole ion trap, the range and efficiency of the ion mass-to-charge ratios captured in the RF quadrupole ion trap may be controlled by, for example, selecting the RF and DC voltages used to generate the quadrupole field, or applying supplementary fields, e.g. broadband waveforms. A collision or damping gas such as helium, nitrogen, or argon, for example, can be introduced via inlet 23 into the ion accumulator 36. The neutral gas provides for stabilization of the ions accumulated in the ion accumulator and can provide target molecules for collisions with ions so as to cause collision-induced fragmentation of the ions, when desired.
The ion accumulator 36 may be configured to radially eject the accumulated ions towards an ion detector 37, which is electronically coupled to an associated electronics/processing unit 24. The ion accumulator 36 may alternatively be configured to eject ions axially so as to be detected by ion detector 34. The detector 37 (or detector 34) detects the ejected ions. Sample detector 37 (or detector 34) can be any conventional detector that can be used to detect ions ejected from ion accumulator 36.
Ion accumulator 36 may also be configured, as shown in FIG. 1B, to eject ions axially towards a subsequent mass analyzer 45 through aperture 28 (optionally passing through ion transfer optics which are not shown) where the ions can be analyzed. The ions are detected by the ion detector 47 and its associated electronics/processing unit 44. The mass analyzer 45 may comprise an RF quadrupole ion trap mass analyzer, a Fourier-transform ion cyclotron resonance (FT-ICR) mass analyzer, an Orbitrap™ electrostatic-trap type mass analyzer or other type of electrostatic trap mass analyzer or a time-of-flight (TOF) mass analyzer. The analyzer is housed within a high vacuum chamber 46 that is evacuated by vacuum port 43. In alternative configurations, ions that are ejected axially from the ion accumulator 36 may be detected directly by an ion detector (47) within the high vacuum chamber 46. As one non-limiting example, the mass analyzer 45 may comprise a quadrupole mass filter which is operated so as to transmit ions that are axially ejected from the ion accumulator 36 through to the detector 47.
FIGS. 1A-1B illustrate two particular examples of mass spectrometer systems in which ion transport devices may be used to deliver ions from an atmospheric or near-atmospheric ion source into a vacuum chamber. Such ion transport devices may be of various types including, for example, the ion transport device illustrated in FIG. 2A, well-known ion funnel devices, the improved ion transport apparatus disclosed herein (discussed below), as well as other types. All these ion transport devices may be generally employed in other types of mass spectrometer systems in addition to the systems shown in FIGS. 1A-1B. For example, whereas the systems of FIGS. 1A-1B include an ion accumulator or ion trap (36), other mass spectrometer systems, such as triple-quadrupole mass spectrometer systems, may similarly advantageously employ such ion transport devices (as are known in the art or as described in the present teachings). Instead of employing an ion accumulator or ion trap mass analyzer, triple quadrupole systems (not specifically illustrated in the drawings) instead generally employ a sequence of quadrupole apparatuses comprising: a quadrupole mass filter (Q1), an RF-only quadrupole collision cell (Q2) and a second quadrupole mass filter (Q3). As with the systems illustrated in FIGS. 1A-1B, these mass analyzer components reside in one or more evacuated chambers and, thus, an ion transport apparatus as disclosed herein may be advantageously employed if ions are generated in an atmospheric or near-atmospheric ion source.
FIG. 2A depicts (in rough cross-sectional view) details of an example of an ion transport device 5 as taught in U.S. Pat. No. 7,781,728, which is assigned to the assignee of the instant invention and is hereby incorporated by reference herein in its entirety. Ion transport device 5 is formed from a plurality of generally planar electrodes 38, comprising a set of first electrodes 16 and a set of second electrodes 20, arranged in longitudinally spaced-apart relation (as used herein, the term “longitudinally” denotes the axis defined by the overall movement of ions along ion channel 32). Devices of this general construction are sometimes referred to in the mass spectrometry art as “stacked-ring” ion guides. An individual electrode 38 is illustrated in FIG. 2B. FIG. 2B illustrates that each electrode 38 is adapted with an aperture 33 through which ions may pass. The apertures collectively define an ion channel 32 (see FIG. 2A), which may be straight or curved, depending on the lateral alignment of the apertures. To improve manufacturability and reduce cost, all of the electrodes 38 may have identically sized apertures 33. An oscillatory (e.g., radio-frequency) voltage source 42 applies oscillatory voltages to electrodes 38 to thereby generate a field that radially confines ions within the ion channel 32. According to a preferred embodiment, each electrode 38 receives an oscillatory voltage that is equal in amplitude and frequency but opposite in phase to the oscillatory voltage applied to the adjacent electrodes. As depicted, electrodes 38 may be divided into a plurality of first electrodes 16 interleaved with a plurality of second electrodes 20, with the first electrodes 16 receiving an oscillatory voltage that is opposite in phase with respect to the oscillatory voltage applied to the second electrodes 20. In this regard, note that the first electrodes 16 and the second electrodes 20 are respectively electrically connected to opposite terminals of the oscillatory voltage source 42. In a typical implementation, the frequency and amplitude of the applied oscillatory voltages are 0.5-3 MHz and 50-400 Vp-p (peak-to-peak), the required amplitude being strongly dependent on frequency.
To create a tapered electric field that focuses the ions to a narrow beam proximate the exit 39 of the ion transport device 5, the longitudinal spacing of electrodes 38 may increase in the direction of ion travel. It is known in the art (see, e.g., U.S. Pat. No. 5,572,035 to Franzen) that the radial penetration of an oscillatory field in a stacked ring ion guide is proportional to the inter-electrode spacing. Near entrance 27, electrodes 38 are relatively closely spaced, which provides limited radial field penetration, thereby producing a wide field-free region around the longitudinal axis. This condition promotes high efficiency of acceptance of ions flowing from the ion transfer tube 15 into the ion channel 32. Furthermore, the close spacing of electrodes near entrance 27 produces a strongly reflective surface and shallow pseudo-potential wells that do not trap ions of a diffuse ion cloud. In contrast, electrodes 38 positioned near exit 39 are relatively widely spaced, which provides effective focusing of ions (due to the greater radial oscillatory field penetration and narrowing of the field-free region) to the central longitudinal axis. A longitudinal DC field may be created within the ion channel 32 by providing a DC voltage source 41 that applies a set of DC voltages to electrodes 38.
In an alternative embodiment of an ion transport device, the electrodes may be regularly spaced along the longitudinal axis. To generate the tapered radial field, in such an alternative embodiment, that promotes high ion acceptance efficiency at the entrance of the ion transport device as well as tight focusing of the ion beam at the device exit, the amplitude of oscillatory voltages applied to electrodes increases in the direction of ion travel.
A second known ion transport apparatus is described in U.S. Pat. No. 6,107,628 to Smith et al. and is conventionally known as an “ion funnel”. FIG. 3 provides a schematic depiction of such an ion funnel apparatus 50 in both a longitudinal cross-sectional view and end-on view as viewed along the axis 51. Roughly described, the ion funnel device consists of a multitude of closely longitudinally spaced ring electrodes, such as the four illustrated ring electrodes 52a-52d, that have apertures that decrease in size from the entrance of the device to its exit. In FIG. 3 as well as in subsequent drawings, different patterns on the representations of the various different electrodes are provided only to aid in visual distinguishing between the various electrode representations and are not intended to imply that the electrodes are necessarily formed of differing materials. The apertures are defined by the ring inner surfaces 53 and the ion entrance corresponds with the largest aperture 54, and the ion exit corresponds with the smallest aperture 55. The electrodes are electrically isolated from each other, and radio-frequency (RF) voltages are applied to the electrodes in a prescribed phase relationship to radially confine the ions to the interior of the device.
The relatively large aperture size at the entrance of the ion funnel apparatus (FIG. 3) provides for a large ion acceptance area, and the progressively reduced aperture size creates a “tapered” RF field having a field free zone that decreases in diameter along the direction of ion travel, thereby focusing ions to a narrow beam which may then be passed through the aperture of a skimmer or other electrostatic lens without incurring a large degree of ion losses. Generally, an RF voltage is applied to each of the successive ring elements so that the RF voltages of each successive element are 180 degrees out of phase with the adjacent element(s). A DC electrical field may be created using a power supply and a resistor chain (not illustrated) to supply the desired and sufficient voltage to each element to create the desired net motion of ions down the funnel. The depiction in FIG. 3 of the ion funnel known in the art is very schematic. Practical implementations of this device often include a first portion of the device that has a plurality of spaced-apart ring electrodes 52a all having the same large inner diameter and a second portion of the device having the ring electrodes 52a-52d, etc. whose inner diameters taper down gradually so as to focus the ions towards the central axis and the smallest orifice at the exit end 55. The first portion is located on the side where the ions enter the device. In operation, the ion-laden gas emerging from the atmospheric pressure enters, by means of one or more ion transfer tubes or orifices, into the first portion of the device where it emerges at high velocity and undergoes rapid gas expansion. The length of the first portion of the device provides a minimum lateral distance between the ion transfer tubes (or other entrance orifice or orifices) and the tapering-diameter second portion within which the forward velocity of the ion laden gas is lowered by collisions with background gas. When the forward velocity of the ion laden gas has sufficiently been lowered, it becomes possible to manipulate the ions with radio frequency electric fields with low enough amplitudes to be below the Paschen breakdown limit, and preferentially guide the ions towards the exit end 55. Refinements to and variations on the ion funnel device are described in (for example) U.S. Pat. No. 6,583,408 to Smith et al., U.S. Pat. No. 7,064,321 to Franzen, EP App. No. 1,465,234 to Bruker Daltonics, and Julian et al., “Ion Funnels for the Masses: Experiments and Simulations with a Simplified Ion Funnel”, J. Amer. Soc. Mass Spec., vol. 16, pp. 1708-1712 (2005).
As noted in the foregoing discussion, various conventional mass spectrometer system designs use an ion transfer tube to transport solvent laden cluster ions and gas into a first vacuum chamber of a mass spectrometer where either an ion funnel or an alternative type of stacked ring ion guide is used to capture the ion cloud from the free jet expansion. As the high velocity gas enters the ion funnel or stacked ring ion guide, ions are propelled by the co-expanding gas predominantly in the forward direction and are controlled and guided by the RF field towards a central orifice at the exit end of the ion funnel or stacked ring ion guide. The inventors have observed that, as the high velocity gas impacts solid components of such ion transport apparatuses, it leaves a distinctive mark comprising a residue of contaminants that build up on certain portions of the electrodes. Over time, the continued build up of these deposited contaminants can cause electrical arcing across the closely spaced electrodes. As a result, mass spectrometers that employ such ion transport devices require occasional time-consuming disassembly and cleaning of these devices.
Traditionally ion funnels or stacked ring ion guides are constructed from a stack of parallel plates (metal or metalized around the orifice of an FR-4 printed circuit board), each plate having an orifice. In the case of ion funnels, the orifices are of decreasing diameter in the direction from the apparatus entrance to the apparatus exit. The outside edges of the plates are generally of quasi constant dimensions, shaped, for example, circularly, square, or some combination thereof. In some designs, also solid spacers are inserted between the plates to keep them apart.
As a result of this multiple parallel plate construction, high velocity gas from the expansion out of the ion transfer tube cannot easily escape the ion transport apparatus so that it can be pumped away. Consequently, gas pressure may increase to an undesirable level in the chamber containing the ion transport device. This problem may be especially serious in the case of ion-funnel-type ion transport apparatuses, since the projection of the funnel along its symmetry axis shows or presents only the orifice at the end as an opening for escaping gas. The conductance between successive funnel electrodes is oriented close to perpendicular to the direction of the expansion, which creates a relatively high pressure area in the funnel. This problem has been exacerbated in recent years as the throughput of transfer tubes has been gradually increased via the use of “multi bore capillaries”, larger diameter bore, or “letter box” type transfer tubes. This has negatively impacted the ion transmission efficiency of the ion funnel or stacked ring ion guide and, although operation at higher RF frequencies can help to alleviate this problem, reducing the pressure within the device itself is a better solution if one wants to keep increasing the throughput from the atmospheric pressure ionization source. In addition, the robustness of the device (as defined in the number of plasma shots needed before cleaning) is limited by the beam impacting on the electrodes opposite the transfer tube.